It is well known that one of the critical factors governing the efficiency of a turbine, particularly the turbine of an axial flow gas turbine engine, is the magnitude of the clearance between the radial outer tips of the turbine rotor blades and the radially inner surface of the casing which surrounds them. If the clearance is too large, there can be a leakage of turbine gases between the turbine blade tips and the casing resulting in turn in a reduction in turbine efficiency. It is of course possible to build the turbine in such a manner that the clearance is very small. However the thermal changes which inevitably occur during gas turbine engine operation result in the variation of the clearance. If the clearance is too small, there is a very real danger of the turbine blade tips actually making contact with the casing.
Several approaches have been made in the past to the control of turbine blade tip clearance by blowing hot or cold air on to the external surface of the turbine casing so as to control its temperature and thereby in turn its thermal expansion. For instance in UK Patent No. 1248198 there is described a turbine blade tip clearance control system in which the clearance between the turbine blade tips and surrounding casing is measured and the resultant measured value is used to control a device which directs hot or cold air on to the casing. The actual air temperature is selected such that the casing thermally expands or contracts to such an extent that the tip clearance is maintained at a pre-selected value. Similarly in UK Patent No. 1561115 there is described a clearance control system in which cool air is directed on to the turbine casing in order to reduce the rate at which the casing thermally expands. The actual amount of cooling directed on to the casing is controlled in accordance with an engine operating parameter.
Although such techniques for controlling turbine blade tip clearance can be effective, it is sometimes difficult to ensure that thermal expansion and contraction of the turbine casing is sufficiently large as to provide an optimum turbine blade tip clearance under the majority of engine operating conditions.
It is an object of the present invention to provide means for controlling turbine blade tip clearance in which an optimum clearance is achievable under the majority of engine operating conditions.